maybe you could start here.

Last year I did a 4 month unit on telephones.  The kids learned how to use telephones, and the history of telephones.  They constructed their own non-working models of vintage phones.  They de-constructed telephones of various vintages, then compared and contrasted the inner workings of the telephones.  Using an online repair manual, the kids reconstructed parts of their phones.  They explored sound using a funnel phone, and can/string telephones.  We had a telephone repair person come in to show the special equipment, then he helped the kids create a working phone system within our classroom.  The kids surveyed people about the type of phone they had ever used: rotary dial, corded and cordless telephones, then analyzed the results.  If you decide you want to do this project, and want further assistance, I can be contacted at vkerr@kcmsd.net.  I'm attaching some of the resources we used.